Question 285517
The wording is what makes it hard. Why cannot Mathematicians consider learning English!
Initially, the sales are $500 from which there is a profit of $100
Afterwards there are sales of $700, from which there is a profit of $140.
Assuming that the advertising is an extra cost of $20, the extra profit is $20 less than the $140 - $100 difference, or $20.
If however, the rate of profit stays the same, this might be construed to mean that the advertising is now included in the costs of sales and does not have to be considered seperately. In that case the profit increase is $40 per day.
Either interpretation is possible when the question is written in this way.